Compressed air energy storage (CAES) is a proven means of storing mechanical or, via an electric generator, electrical energy for subsequent use on a very large scale. Existing commercial CAES facilities store the compressed air in naturally occurring underground geological formations, and obtain the heat needed to fully recover the stored energy by burning natural gas. The former feature avoids the considerable cost of the pressure vessels that would otherwise be needed to confine the compressed air, but forces these CAES facilities to be located near to suitable geological formations. The latter feature allows the stored energy to be recovered efficiently and at an acceptably high power level, but adds to the cost of operating the facility and results in the emission of carbon dioxide, a persistent pollutant and greenhouse gas.
A number of approaches to making CAES economical without any net carbon emissions and/or less limiting site restrictions are under development. There are two ways to avoid the carbon emissions. The first is to take the heat needed to recover the stored energy upon expansion from a carbon-neutral source, for example the surrounding atmosphere or ground, solar thermal hot water, biomass or a waste stream. The second is to store the heat produced by compressing the air for reuse upon expanding it. These two forms of carbon-neutral CAES are not separated by a sharp line; for example the heat of compression may be stored in a water reservoir, and the heat lost during storage made up for using one of the aforementioned heat sources.
There are likewise two, not-necessarily exclusive, ways to avoid site restrictions of existing CAES facilities. The first is to reduce the cost of the pressure vessels by various means. The most widely applicable means is to use high pressures (typically of order 200 atmospheres). This reduces the amount of steel or other material required to store a given amount of energy as compressed air (logarithmically with pressure in the isothermal model), and hence the cost per unit of stored energy as well. The cost can also be lowered by using buried pipes in a remote or restricted area, in which case the overpressure safety factors required by ASME (American Society of Mechanical Engineers) regulations are much smaller than those for unburied tanks (1.4 instead of 3.5). Still another means of cutting the cost is to confine the air using a less expensive material than steel.
Unfortunately, inexpensive building materials such as concrete lack sufficient strength in tension, while modern high-tensile strength materials such as carbon-fiber resins still cost considerably more per unit strength. Viable alternatives include artificial underground chambers, which may be excavated in certain kinds of rock for a reasonable cost, or confining the air in fabric containers underwater at a depth where the hydrostatic pressure equals the desired operating pressure. These latter possibilities have their own site restrictions, although much less severe than those of suitable naturally occurring geological formations.
The second class of methods for confining the air without severe site restrictions is chemical rather than mechanical. One such approach is to store the air at ambient pressure but as a liquid at cryogenic temperatures (ca. −160° C.). Another is to adsorb the air in a nano-porous solid material such as a zeolite. The amount of air adsorbed may be controlled by adjusting the temperature of the material rather than the pressure over it, specifically by cooling it to make it adsorb air and heating it to desorb the air. With common zeolite materials such as 13X (NaX), the temperatures and pressures required for this Adsorption-Enhanced Compressed Air Energy Storage (AE-CAES) are quite mild, making this a particularly promising albeit still relatively undeveloped form of CAES.
The main challenge in designing a cost-effective AE-CAES system is that several times more heat must be moved around over the course of the storage cycle than in most other approaches to CAES. This includes the sensible heat in the adsorbent and the latent heat of adsorption, in addition to the heat of compression.